go.crypto: revert 7f5a59ff6b43.

This change reverts https://golang.org/cl/10113043/ because
some folks are stuck on 1.0 till 1.1.1 comes out.

R=golang-dev
CC=golang-dev
https://golang.org/cl/10151043

ssh/client_auth.go

@@ -286,6 +286,7 @@ func (p *publickeyAuth) confirmKeyAck(key interface{}, t *transport) (bool, erro
 			return false, UnexpectedMessageError{msgUserAuthSuccess, packet[0]}
 		}
 	}
+	panic("unreachable")
 }
 
 func (p *publickeyAuth) method() string {
@@ -324,6 +325,7 @@ func handleAuthResponse(t *transport) (bool, []string, error) {
 			return false, nil, UnexpectedMessageError{msgUserAuthSuccess, packet[0]}
 		}
 	}
+	panic("unreachable")
 }
 
 // ClientAuthKeyring returns a ClientAuth using public key authentication via